The invention concerns a superconducting high-field magnet system for a high-resolution magnetic resonance spectrometer, comprising a substantially cylindrical cryostat with an axial room temperature bore for receiving a sample, and a radio-frequency transmitting and detecting system, with a magnet coil which is superconductingly short-circuited during operation and which is disposed at a low temperature in a region within the cryostat to surround the sample in the room temperature bore and to generate a homogeneous, temporally stable magnetic field at the sample location during operation, wherein the magnetic field meets the requirements for obtaining a high-resolution magnetic resonance spectrum.
High-resolution nuclear magnetic resonance (NMR) spectrometers must have magnetic fields with good temporal stability in addition to an extremely high homogeneity over the sample volume. Towards this end, the superconducting main coil of the magnet is superconductingly short-circuited during operation. The properties of the superconducting short-circuit switch, the quality of the superconducting coil wires and superconducting joints between individual wire sections of the coil must therefore meet highly stringent requirements. During short-circuit operation, decay times of the superconducting coil current of several 10,000 years must be guaranteed.
Temporal fluctuations of the magnetic field at the sample location may be compensated for using a so-called lock system. Towards this end, the spectrometer measures a separate NMR signal of a lock substance (i.a. deuterium) in a frequency band provided for this purpose, and its frequency is stabilized via a feedback circuit using a small resistive compensation coil (lock coil) in the room temperature bore of the magnet system.
A superconductingly short-circuited magnet coil keeps the magnetic flux through its bore constant, i.e. the superconducting current changes spontaneously in response to e.g. an external disturbance field, such that the total flux through the coil does not change. This does not generally mean that the field in the working volume remains absolutely homogeneous and constant, since the local field distribution of a disturbance and of the main magnet coil do not coincide. Prior art has proposed compensation of these deviations through the design of the main coil geometry, additional superconducting coils or through active control measures (U.S. Pat. No. 4,974,113; U.S. Pat. No. 4,788,502; U.S. Pat. No. 5,278,503).
High-resolution superconducting NMR magnets generally use superconducting shim coil sets to initially homogenize the field at the sample location. During operation, the individual coil sets are charged with a correction current and are superconductingly short-circuited. The shim coil sets may also comprise a so-called B0 coil which can generate a sufficiently homogeneous, small additional field at the sample location. It is thereby possible to finely and precisely adjust the field or the proton frequency to a predetermined value without opening the superconducting circuit of the main coil. Moreover, it has been known for some time that, within certain limits, a main coil drift can also be compensated for using the short-circuited B0 coil. Towards this end, the B0 coil must be disposed and dimensioned such that the field decay of the main coil induces a counter current in the B0 coil which causes the field at the sample location to remain constant. This method is limited in that the current through the B0 coil must not become excessively large. The maximum current may be limited by the wire which is used. In any event, the contribution of the (low homogeneity) B0 coil must remain sufficiently small such that the field homogeneity throughout the sample is not impaired. Moreover, in case of a quench, the B0 coil could be overloaded and destroyed in consequence of the required inductive coupling between the B0 coil and the main coil. Corresponding protective means must be introduced, which result in additional expense.
The production of superconducting high field magnets for high-resolution NMR spectrometers (and also ion cyclotron resonance (ICR) spectrometers) has reached a very high level of quality and reliability. The currently used wire materials are multi-filament wires of NbTi or Nb3Sn filaments in a copper matrix (with common, conventional modifications). Corresponding wires can be obtained e.g. from the company European Advanced Superconductors GmbH & Co KG, Hanau. The problems involved with the production of superconducting joints and switches between coils or coil sections made from these wires are also solved in the above-mentioned NMR spectrometers (see e.g. EP 0 459 156 B1).
For the highest desired magnetic fields—currently in the region of around 23 Tesla—one has started to replace conventional low temperature superconducting (LTS) wires in the highest field section with wires (bands) of ceramic, high-temperature superconductors (HTS) which permit (at a low temperature of approximately 4 K or 2 K) higher critical field strengths and current densities. The initial problem of producing wires from these materials which meet the high NMR requirements also seems to be solved. However, the problem with the superconducting joints and switches between and to these wires remains unsolved. There are different approaches which do, however, not form part of the present invention. It is still unclear which approaches will ultimately be successful. However, it is, in principle, much easier to produce drift-less joints between wires of the same material or the same material class than between different materials, in particular, between HTS and LTS conductors.
Accordingly, a highest-field NMR magnet coil will therefore consist of a series of sections with NbTi wire, sections with Nb3Sn wire (or further developments thereof) and at least one section with HTS wire (or band). For this reason, the use of joints is desired which join NbTi wire and NbTi wire without drifts (solved), those between NbTi wire and Nb3Sn wire (solved), those between Nb3Sn wire (and/or NbTi wire) and the HTS conductor (currently not solved) and optionally between identical and/or different HTS conductors (currently solved on a laboratory scale and considerably easier). I.a. bismuth conductors, generally in the form of a band-shaped multi-filament wire in a silver matrix, YBCO as “coated conductor” or conductors produced in accordance with the “powder-in-tube-method” are currently being examined as HTS conductors. Further variants are feasible which exist at the moment only on a laboratory scale.
The recently discovered conductor MgB2 could be used in the LTS and also in the HTS circuit, assuming that it is compatible with the corresponding joints.
Due to the fact that the problems with joints are only partially solved, there are currently no magnets for high-resolution NMR with HTS partial coils.
For this reason, there is a need for a superconducting magnet system of the above-mentioned type which produces higher magnetic fields at the sample location thereby still meeting the NMR criteria with, in particular, the prevention or at least minimization of drifts.